Big Bang Predictions - Exploring Energy Density & Expansion Rate

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In summary, the author is trying to understand how it is possible to make predictions about the energy density of early universe based on the expansion rate of the universe, but they don't need to use the Friedmann equation.
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electerr
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Hi,

I am trying to understand how it is possible to make predictions about the energy density of early universe using the freidman equation if the expansion rate of the universe has not been constant throughout history. As I understand it there are three main variables in the freidman equation, the energy density, the Hubble constant squared (the expansion rate of the universe) and the shape of space (K). We know today that space is flat (or very close to flat) but in order to calculate anything with the other two variables one of them must also be known. I thought that the expansion rate was first thought to accelerate, decrease, and is now accelerating again and that as the universe has expanded that radiation particles has become redshifted and lost energy changing the energy density. With these two unfixed variables how can you say anything certain about either one? As you can probably guess this is all very new to me so maybe I am missing something very obvious but maybe I would be thankful if someone can try and clue me in...?
 
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The Friedmann equation comes from the 00 component of the field equations, but we also have the acceleration equation which comes from the trace. This gives a set of two equations in two variables.
 
  • #3
electerr said:
Hi,

I am trying to understand how it is possible to make predictions about the energy density of early universe using the freidman equation if the expansion rate of the universe has not been constant throughout history.
No need to use the Friedmann equation. Just use conservation of stress-energy and the scale factor.

Of course, if you want this as a function of time, you have to use the Friedmann equation, but if doing it as a function of either redshift or scale factor is your concern, there's no need.
 
  • #4
Ok, I didn't know about the separate Friedmann acceleration equation. Thanks for the help!
 

1. What is the Big Bang Theory?

The Big Bang Theory is a scientific explanation for the origins of the universe. It proposes that the universe began as a singularity, a point of infinite density and temperature, approximately 13.8 billion years ago. It then expanded and cooled, eventually leading to the formation of galaxies, stars, and planets.

2. What are the predictions of the Big Bang Theory?

The Big Bang Theory predicts several key aspects of the universe, including the cosmic microwave background radiation, the abundance of light elements such as hydrogen and helium, and the expansion of the universe. It also suggests that the universe is expanding at an accelerating rate due to the presence of dark energy.

3. How does energy density affect the expansion rate of the universe?

According to the Big Bang Theory, the energy density of the universe plays a crucial role in determining the expansion rate. In the early universe, when the energy density was high, the expansion rate was also high. As the universe expanded and cooled, the energy density decreased, leading to a slower expansion rate.

4. What evidence supports the predictions of the Big Bang Theory?

There is a wealth of evidence that supports the predictions of the Big Bang Theory. This includes the discovery of the cosmic microwave background radiation, the abundance of light elements in the universe, and the observation of an expanding universe through the redshift of galaxies. Additionally, the recent discovery of gravitational waves from the collision of two neutron stars is further confirmation of the Big Bang Theory.

5. Are there any challenges to the Big Bang Theory?

While the Big Bang Theory is the most widely accepted explanation for the origins of the universe, there are some challenges and unanswered questions. These include the mystery of dark matter and dark energy, the singularity at the beginning of the universe, and the horizon problem, which refers to the uniformity of the cosmic microwave background radiation. Scientists continue to study and explore these challenges in order to refine and further understand the Big Bang Theory.

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